For at least decades and perhaps centuries, wood has been the preferred material for making doors and windows. When making, for example, a door having window panes, the horizontal top and bottom door structural portions (called "rails") and the vertical left and right door structural portions (called "stiles"), were often made of homogeneous unlaminated lumber.
With the diminution of stands of finer timber and with the advent of improved bonding techniques, manufacturers and builders have turned at least in part to so-called composite wood structures for rails and stiles. And it is believed that vinyl-clad composite wood structures are being used for window frames.
Composite wood structures are not made of homogeneous lumber. Rather, such structures often include planar layers of relatively thin strips of wood bonded together. Such layers, hidden from view in the finished product, may be cut from lesser-grade lumber. Examples of such a layered product are depicted in U.S. Pat. Nos. 5,071,688 (Hoffman); 4,743,484 (Robbins) and 5,234,747 (Walser et al.). Products depicted in the latter two patents are often referred to as "laminated veneer lumber" or simply "LVL." LVL is usually made of thin veneer-like planar portions or sheets oriented with their grains parallel and bonded together.
And wood strips are not the only material used to make door rails and stiles. Composite particle board (or simply composite board), often of the high density type, may be bonded to either side of an LVL core. The resulting "intermediate" product is used by door manufacturers in making rails and stiles for doors. And a decorative wood veneer may be bonded to one of the high-density layers. For aesthetic reasons, such veneer is intended to be on the interior side of the door and has applied thereto the finish coat of stain, varnish, paint or the like which is visible from the interior of the building. And veneer may be used on the exterior side of the door for the same purpose.
U.S. Pat. Nos. 1,387,011 (Ratcliff); 1,448,180 (Atwood et al.); 4,543,284 (Baum) and 5,225,264 (Kato et al.) all depict examples of efforts, not necessarily useful in making doors and windows, to "waterproof" wood structures. The Ratcliff and Atwood et al. patents involve multilayer structures including wire mesh. Such structures are wholly unacceptable for making doors and windows, at least because wire mesh damages wood cutting tools.
U.S. Pat. No. 4,146,662 (Eggers et al.) depicts a door having a solid particle board core with its exterior surfaces covered with a phenolic resin-impregnated overlay paper. The resulting door is said to have twice the warp resistance of standard wood doors.
While doors of the types described above have been generally satisfactory for their intended purposes, they are distinctly unsatisfactory in certain circumstances. So-called "weather doors," those having one side of the door exposed to the outdoor ambient, are often subjected to temperature and humidity conditions on one side which are disparate from those on the other. Moisture migration results and such migration is usually attended by warpage of some degree.
Understanding the phenomenon of warpage due to moisture migration will also be aided by the following. When composite wood structures are made into doors, the normal moisture content of the wood is on the order of 6-12%. Such moisture content is relatively evenly distributed through the thickness of the door. Moisture swells wood fibers but since such moisture is evenly distributed, such swelling is substantially uniform and little if any warpage occurs. However, if a significant moisture "gradient" develops across the thickness of the door, serious warpage may result.
In geographical areas having extremes of temperature and humidity like those found in the arid southwest of the United States, e.g., very hot and dry on the outside and, because of air conditioning or evaporative cooling, cool and moist on the inside, often cause a door to warp beyond acceptable limits. This is so since the moisture gradient across the thickness of the door may range from, say, 4% on one side to, say, 14% on the other side, a gradient of about 10%.
Door lower rails seem particularly susceptible to such warpage, perhaps because they are not well protected by roof overhang and are in the direct rays of the sun. And in larger doors, e.g., 8 foot doors, stile warpage can be a significant problem, as well.
Warpage above about 0.100 inches over the width of a door which is nominally 4 feet wide is often considered excessive and unacceptable by door manufacturers. For a sliding "glider" or patio door, excessive warpage manifests itself in a door which will not slide freely, if at all. And excessive warpage in a vertically-hinged door can cause such door to fail to seal or be difficult or impossible to close or open.
A composite wood structure which is effective in reducing door and window warpage to acceptable limits, even under interior/exterior extremes of temperature and humidity, would be an important advance in the art.